Acoustic logging tools with acoustic attenuating structure



June 22, 1965 E, 1, MQsER ETAL 3,190,388

ACOUSTIC LOGGING TOOLS WITH ACOUSTIC ATTENUATING STRUCTURE Filed may 1e. 1961 s sheets-sheet 1 nf 2/ n Z 7 ff- 36e- 24 jab i A f-z da K Tf1' j? TT A 4/ NJW/cr 42 l 1 Z l MHH] 1.

fdc 35 IN VEN TOR.

,Amir

June 22, 1965 E. l. MosER ETAL. 3,190,388

ACOUSTIC LOGGING TOOLS WITH ACOUSTIC ATTENUATING STRUCTURE Filed May 16, 1961 3 Sheets-Sheet 2 fz/l y r//vf m l M V Var/165 Geo/*ge H. P41/due fav/ward f MaJef INVENTOIL ATTO/@VZV June 22, 1965 E. l. MOSER ETAL ACOUSTIC LOGGING TOOLS WITH ACOUSTIC ATTENUATING STRUCTURE Filed May 16, 1961 @OQ u D QJ 3 Q Tf o jx i'oo@ Q00: 'o @0. o@ 0: o0@` EOC) Oi klxv' 3 Sheets-Sheet 3 fc4/vara f /WaJer INVENTOR.

United States Patent Office @,lghgg Patented .lune 22, 1965 3,190,388 ACGUSTIC LOGGING TLS WITH ACOUSTHC ATTENUATDIG STRUCTURE Edward I. Moser andV George H. Pardue, Houston, Tex., assignors'to SchlumbergerWell Surveying Corporation,

Houston, Tex., a corporation of Texas f VFiled May 16, 1961, Ser. No. 110,471 6 Claims. (Cl. 181-5) This invention relates to exploratory tools for use in wellbores and, more particularly, to acoustic well logging tools which probe the media surrounding a well bore with pulses of acoustic energy.

An acoustic well`logging tool is generally cyliudrically shaped and suitably sized for passage through auid illed well bore. Normally, the tool carries two or more transducers which are disposed and secured at a iixed distance from one another. In a typical acousticV tool 'having three transducers, one of the transducers serves as a transmitter of sound'waves while' the remaining transducers serve as receivers of sound waves. The receivers are spaced from one another at a predetermined 'distance and are disposed to one side of the transmitter along the longitudinal axis of the tool. In operation, the transmitter in the tool is electrically actuated'periodically "to emit-pulses of acoustic energy (or pressure waves) `which propagate outwardly from the transmitter with a velocity dependent upon the media traversed by the energy. The arrival of the acoustic energy at the successively positioned receivers is detected to trigger a timing circuit in the tool which functions to ascertain the time required for a givennpulse of acoustic energy to travel the predetermined distance between the two receivers. Prom a time measurement obtained by the timing circuit, a composite velocity of media traversed by an acoustic pulse can be determined. In turn, composite velocity indications can be correlated to specific Itypes of earth formations or other media. Indications of travel time or velocity obtained in open boreholes can also be related to the porosity of earth formations. If

desired, the'timing function can be accomplished with "only a single transmitter and receiver.

Acoustic energy as above discussed can be generated or intercepted by a piezoelectric or a magnetostrictive transducer in a well known manner.

In a typical open borehole, an acoustic tool is, commonly spaced from the wall of the well bore so that the `emitted acoustic wave energy Vor pressure pulses are irst fluids in the well is generallyV in the neighborhood of 5000 feet per second, while the characteristic velocity of wave motionthrough the earth formations may vary from V5000 feet" perfsecond to 25,000' feet per second. Thus, the portion of the acoustic wave energy transmitted into the formations generally travels at a higher velocity than the corresponding portion of the wave energy traveling in the well bore uid. Because of this, the portion of the wave `energy traveling throughearth formations reaches a receiver prior to the time that the portion of the -acoustic "wavenen'ergy traveling through thef fluids does. 4It is this feature of higher formation velocity which permits measurement of the velocity of acoustic energy in earth formations surrounding a borehole.

Typically, each pulse of acoustic energy upon intercepting a receiver transducer generates an electrical signal containing a number of undulations, cycles or vibrations. The time measurement is generally based upon the detecti-on of a given portion or characteristic of an electrical signal developed at the respective receivers for a given traveling pulse of acoustic energy. A commonly used characteristic of a corresponding electrical signal for detecting purposes, for example, is a voltage amplitude value. This is made possible because thel undulations, cycles or vibrations of a typical electrical signal as developed from a typical pulse of acoustic energy generally include, in the first cycle, 4a first peak of a given polarity followed by a` second peakwof an opposite polarity and approximately three times the magnitude of the first peak and, in the second cycle, a third peak with a polarity similar to the iirst peak and about ten times the magnitude of the first peak. Hence, when a selected characteristic voltage amplitude value is exceeded, a detection signal for operating the timing circuit can be developed. The characteristic voltage amplitude value selected for detection purposes is generally such that detection will occur during the firstV cycle of a signal. The `selection of a voltage amplitude characteris-tic of a firstcycle of the `signal to detect the first arrival of the acoustic signal is desirable because the voltage amplitude values of subsequent cycles are generally distorted because of acoustic reflections in the borehole. v

From the foregoing discussion concerning the nature of acoustic wave propagationin an open borehole and timing of such propagation over .a iXed distance, it is apparent that a suitable supporting means for the transducers must be incapable of passing detectable acoustic energy longitudinally between the transducers at a velocity higher than that of the adjacentr earth formations. Obviously, if the supporting means are not so constructed,

`,the receiver circuit would be triggered 'prematurely by the 1' energy from triggering the transducer prior to the earliest arrival of the acoustic energy traveling through earth formations. However, to meet these acoustical inhibiting conditions for acoustically blocking the direct sound path, the support means have been complex and expensive to ymanufacture and have been expensive to maintain and have been lacking in strength qualities for repeated, general field use.

p Accordingly, it is an object of the present invention to provide new and improved acoustic logging tools wherein the support has high strength qualities as well as acoustical attenuation and delay characteristics relative to the transmission of detectable acoustic energy lengthwise of thev support between transducers. Y

A further object of the present invention is to provide new andV improved acoustic logging tools having a relatively high strength and stiffness to withstand the shocks and forces inherently encountered in a logging operation.

p Another object of the present invention is vto provide acoustic logging tools with a support strong in tension to facilitate a fishing or retrieving operation if the tool should become temporarily immovable in the well bore.

A still further object of the present invention is to provide a new and improved support for acoustic logging tools which is constructed of metal with an arrangement such that the support alters the intensity and time of arrival of detectable acoustic energy along its length.

Yet another object of the present inventon is to provide a new and improved support for acoustic logging tools in accordance with the foregoing objects which is relatively inexpensive to manufacture and is durable and reliable in field opertions.

Apparatus in accordance with the present invention includes an elongated, relatively stiff, high-strength metallic tubular member for carrying at least two acoustic transducers in a spaced apart relation. The generally tubular configuration of the member is characterized by recesses or open spaces arranged in a pattern about the periphery of the tubular member and along its length. The pattern arrangement is such that successive pairs of points spaced along generatrices of said member are joined by solid portions of the member to form an acoustic path which is substantially longer in distance than the strtaight line distance along a generatrix between such successive pairs of points. The recesses or open spaces in the tubular member are filled with a material having a high mass and a low modulus of elasticity to alter the intensity and velocity of detectable acoustic energy traveling lengthwise of the member.

The novel features of the present invention are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood by way of illustration and example of certain embodiments when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a view of an acoustic logging tool embodying the present invention;

FIG. lA is an electrical schematic diagram of an operating system for the acoustic logging tool, shown in FIG. l;

FIG. 2 is an enlarged view in longitudinal cross-section taken along line 2 2 of FIG. 1;

FIG. 3 is an enlarged view in cross-section taken along line 3 3 of FIG. 2;

FIGS. 4-6 are front elevation views of portions of housings which display other configurations in which the present invention is embodied;

FIG. 7 is a view of an outer surface ol a portion of a tool as well as a partial view in cross-section in which other configurations embodying the present invention are displayed; and

FIG. 8 is a view in cross-section taken along line 8 8 of FIG. 4.

In the description to follow, it should be understood that the term acoustic energy refers primarily to cornpressional wave energy although it is not intended to exclude acoustic energy such as shear wave energy, etc. Likewise the term characteristic velocity as hereinafter used means the velocity value normally obtained when a pulse of acoustic energy traverses a solid, uninterrupted material object in a straight line path between two fixed points in a given time. The term detectable energy or detectable acoustic energy as used rhereinafter means acoustic energy having a characteristic which is capable of energizing a transducer such that a detecting circuit is responsive to the electrical signal generated in response to the characteristic of the acoustic energy. Apparent velocity, as hereinafter used, means an apparent velocity value normally obtained when a pulse of detectable acoustic energy traverses a material object, constructed and arranged in accordance with the present invention, between two fixed points lengthwise of the object over a time interval other than would be normally expected for the object in its solid configuration. The term acoustic path as hereinafter used means a given solid, substantially continuously connected material providing a medium through which acoustic wave motion may be transmitted.

The present invention is concerned with an acoustic logging tool having an elongated, tubular support constructed of steel for strength and ruggedness. Since the characteristic velocity of acoustic energy in steel is in the neighborhood of 17,000 feet per second and the range of characteristic velocities of the earth formations or media desired to be investigated is from 5,000 to 25,000 feet per second, it will be appreciated why a steel support or housing has heretofore been considered unusable.

Considering first the fact that the characteristic velocity, distance, and time factors are related by the classical expression S=vt so that from any two given values, the third may be reliably calculated, it will be appreciated that for a fixed length of housing, the time factor is inversely related to the velocity factor. Therefore, it would appear that acoustic energy traveling over a fixed length of steel housing which has a characteristic velocity of 17,000 feet per second, would invariably travel over the fixed distance in a fixed time. However, by means of the present invention, the construction of the steel housing can be arranged so that detectable energy travels over a fixed length of housing with an apparent velocity which is considerably less than 17,000 feet per second and in a time interval which is greater than the aforesaid fixed time. In fact, the apparent velocity of a steel housing may be made substantially equal to or less than the characteristic velocity of the fluids in the well bore.

In accordance with the present invention, prime conditions for reducing the characteristic velocity of a length of a tubular support constructed of steel are to substantially eliminate any direct linear path longitudinally of the tubular support and to provide an acoustic interference pattern lengthwise of the housing. Stated another way, the linear continuity of the tubular support in its lengthwise direction is substantially interrupted or disrupted, and by so doing, tortuous acoustic paths are formed. However, the interruption is such that there are remaining longitudinal interconnecting ties which prevent substantial longitudinal exing of the tubular member. This effectively lengthens the path that acoustic energy must follow and also affects the mechanical characteristics of a support by decreasing the longitudinal unit spring rate of the support and the weight per unit length. The discontinuities in the tubular support are plugged with a material having a high mass and low modulus of elasticity to alter the intensity and apparent velocity of acoustic energy lengthwise of the support.

As will now be explained, a derived relationship between the unit spring rate and weight per unit length can give a fair approximation of the apparent velocity of a tubular member with a selected configuration.

From basic physics, it is known that the velocity V in elongated bars and tubes is equal to where E is Youngs modulus of elasticity and p is the mass denslty. It is also known that the weight density d 1s equal to where g is the gravitational constant of 312.3 pounds per sec?. Youngs modulus of elastic-ity E is, of course, equal to Pl E-m (3) :Equation 4 can be rearranged as follows:

al: E V= 12 (Az/e) lgt/g 5) wherein V is velocity in feet per second and where s is the unit spring rate in pounds/in./in., W equals the weight in pounds, and w equals the weight per unit length in pounds/ in.

The above derived relationship is considered applicable for deriving an approximate apparent velocity value for a tubular `member with void spaces or recesses arranged .about its periphery and along its length in such .a manner that a substantially `non-linear or interrupted acoustic path is provided lengthwise of the housing. The apparent velocity derived from the use of this formula is also understood to he the limiting value of velocity which would be obtained as the frequency of the acoustic energy approaches-zero.

The frequency of a pulse of energy produces an effect relative tothe above derived relationship which has not been theoretically determined. However, the following effects have been observed. A predominant frequency value for .a pulse of energy from a given transmitter can be shown to be the resonant frequency of the transmitter and can be easily determined by measurement of the .time period of a cycle. It should be appreciated that a pulse of energy from a transmitter is generally made up of a multitude of frequencies .above and below that of the predominant frequency. The intensi-ty of the frequency components is generally a maximum at the predominant `frequency and decreases for frequencies above and below the predominant frequency.

' The ability of a tubular member to pass sound will Y depend upon the following:

(l) acoustic path length, (2) attenuation tendency or inhibition characteristics,

(3) `spring rate and mass per unit length.

Disregarding pass bands and other frequency sensitive phenomena, itcan be generally stated that the maximum longitudinal Velocity will be determined by the interconnected path length. For a given path width, sound frequencies with a quarter-wave length less than the path lwidth are permitted to pass with relatively low resistance. As the frequency is decreased, the quarter-wave length is increased and the resistance to passage increased, causyging a reduction insignal intensity. This is .also accompanied by a decrease in the speed of'transmission. As the frequency continues to decrease, .the resistance to passage increases and the speed of propagation decreases until the limiting value based on the spring rate and mass per unit length is reached. From this it can be seen that the apparent detectable velocity will fall somewhere between the values obtained by unt-il spring rate-'mass determination and by a determination of the length of the acoustic path, depending upon the 'frequency distribution of the .acoustic path. As would be expected, with a steel tubular member or housing constructed and arranged to nhave a given. apparent velocity calculated by means of the previously derived relationship, a pulse of `energy with, nfor example, a predominant frequency of 30 kc. rvjgenerallytravels through the housing with an .actual "apparentfvelocity which is higher'than a calculated 'appa- 4 rent velocity derived `from thefunit spring rate and weight Aper unit length.

It alsowill be appreciated from the foregoing rela- ,tionship as expressed in Formula 5 that the mass and vvelocity are inversely proportional, i.e. as the mass increases, the velocity decreases.

Turning now to specific illustrations of the present invention, it should be understood that the present invention involves an elongated and generally cylindrical well tool which is to be used in a well bore containing a well `fluid. The tool is adapted to be passed through the well ti bore by means of an armored electrical cable s poo-led on a surface-located winch and is electrically coupled to surface indicating and recording units.

In FIG. 1 there is illustrated an elongated but rigidly constructed acoustic logging tool 2d adapted for passage in the above described customary manner through a well bore (not shown) by means of an armored electrical cabie 2i and winch (not shown) which is situated at the earths surface. The tool 2d includes an upper, tubular cartridge or housing 22 and a lower, tubular housing 23, both of which are preferably constructed of steel. A single centralizer support device 24 disposed at the center of gravity of the tool E@ may be employed if the tool is to be centered in a well bore. Alternatively, two or more centralized supports disposed along the length of the tool may be employed if so desired. Also, if desired, a caliper device 25 for measuring the diameter of the well bore may be coupled to the tool 2), for example, at its lower end as shown in the drawing.

Within the upper cartridge 22 are electronic components and circuits to periodically actuate the transmitter T and to perform the measuring function in response to signals from the receivers El and R2. The 4electronic circuits are coupled via cable 21 to conventional surface indicating and recorder instruments (not shown). Briefl the measuring function may be accomplished by circuits as shown in FIG. 1A wherein a keying circuit 26 periodically triggers the transmitter T .to emit a pulse of acoustic energy. The keying circuit also conditions a pulse generator 27 for operation after a predetermined time interval which is fixed by a delay gate 26. The predetermined time interval is, of course less than the time required for an emitted pulse of acoustic energy to reach the receiver R1. The keying circuit also provides a reset pulse to reset a time to voltage circuit 26a. At the .time the iirst receiver R1 senses the acoustic energy emitted by the transmitter T, a characteristic of the electrical signal developed by .the lreceiver R1 is used to trigger the pulse generator 27 to produce an output pulse. rEhe output pulse of generator 27 triggers a multivibrator 28 into operation and also operatively conditions a pulse generator 27a for operation by means of a gate circuit 27". Thereafter the acoustic pulse arriving at receiver R2 similarly triggers the puise generator 27a to produce an output pulse which triggers the multivibrator 23 into an inoperative condition. The time interval At between the output pulses of the pulse generators 27 and 27a is converted by a time to voltage lcircuit 2da into a voltage signal for transmission to the surface instruments via the cable 2li.

As shown in FIG. l, the lower, tubular housing 23 includes three illustrative and similarly formed transducer sections identified by the number 3) and differentiated from one another by the letters a, b and c. Transkducer sectionsl 30 may be separated from one another by identically formed acoustic inhibiting sections identitied by .the number 3l and differentiated from one another by the letters a and b. In general, each transducer section 3d is constructed and arranged to permit and to facilitate the travel of acoustic energy in a generally radial pattern between a transducer within the tubular housing and the fluids or muds (not shown) in the well bore which are exterior of the tubular housing. Also, in general, each acoustic inhibiting section 31 is constructed and arranged so as to increase the apparent time required for detectable energy to travel over the portions of the housing between transducer sections Sti.

As schematically shown in FIG. 2, exemplary magneto-strictiveV transducers 35 and 35a can be secured in any convenient manner to a tubular support rod .34 having a low velocity characteristic. For example, rod 3ft can be constructed of Teflon, which has a charac- .teristic velocity of 4400 feet per second. While not shown, the electrical conductors for the exemplary transducers 35 and 35u may be passe through openings (not shown) in rod 34 to the electronic cartridge housing 22.

Transducers 35 and 35a are conventional in the art and need not be further described. The support rod 34 can be connected (not shown) in any suitable and convenient manner relative to the lower housing 23 so that the transducers 35 and 35a are disposed in a generally central position relative to the cross section of the lower housing 23 and generally in a central position relative to the length of a transducer section 30. Alternatively, the transducers could be directly attached to the interior of the housing in any suitable manner if so desired.

A transducer section 30, as illustrated in FIGS. 1 and 2, comprises a plurality if openings 36 in the housing 23, which are generally rectangularly shaped lengthwise of 4the housing and are equidistantly spaced from one another about the periphery of the housing. The width of an opening 36 is defined between parallel and longitudinally extending side surfaces and is generally equal to the width of the sections or portions of housing disposed between adjacent openings 36. The length of an opening 36 is defined between upper and lower inside end surfaces 36e, 36d, which slope inwardly towards one another from the outer surface of the housing 23 to its inner surface. The length of the openings 36 generally should be equal to or greater than the longitudinal dimension of a transducer within the housing. Since a typical magnetostrictive transducer has an appreciable longitudinal dimension (2" to 3 in a typical instrument) the openings 36 are illustrated as elongated in a direction lengthwise of the housing. The sections of the housing between the openings 36 provide excellent stress bearing qualities. Preferably, there are from 8 to 12 such openings 36 spaced about the periphery of the housing for a magnetostrictive transducer which emits primarily radial pressure Waves. This range of openings has been found to provide an efficiency of transmission of sound radially :from or into the housing, which ranges from 98% to virtually 100%.

Each acoustic inhibiting section 31 is constructed and arranged to provide void or recessed spaces about the periphery and along the length of the housing 23 which form a generally tortuous configuration. Hence, acoustic -wave transmission lengthwise of the housing occurs by traveling tortuous paths which extend `between successive points spaced along a generatrix of the lower housing 23 and which are greater than the straight line distance between a successive pair of points.

The invention as illustrated in FIGS. l3 involves the steel tubular housing 23 in which inner and outer spiralling generally U shaped or semi-circular grooves 41 and 42 have been cut. Each of the grooves spirals in a similar manner and has the same pitch. The grooves 41 and 42 are, however, displaced at 180 relative to one another. (Note view from a horizontal cross-section as in FIG. 3.) The depth of the grooves 41 and 42 is such that the lowest portion of each groove extends beyond a mid-point between the inner and outer walls of the housing 23. In this manner, a linear path lengthwise of the housing is interrupted. The spacingfbetween the inner and outer grooves lengthwise of the housing is made less than one quarter wave length of the principal frequency of the acoustic energy. The pitch of the grooves is made such `that energy traveling in a helical path through the solid portion separating the grooves will be delayed by the extra path length created.

More specifically, in explanation of the above described arrangement, there are two primary paths of sound transmission which may exist. Considering a first path as the continuous helical strip of the housing it will be appreciated that an acoustic impulse will travel along this helical strip at the velocity of sound in steel. The time required for an acoustic impulse to travel such a helical path is dependent upon the helix angle of the strip. To calculate an appropriate helix angle the following example is provided: let t1 be equal to the time required for an acoustic impulse to travel a length l1 along the helix at a velocity c where c is equal to the characteristic velocity of steel. Let t2 Ibe equal to the time required for an acoustic impulse to travel a distance l through the adjacent media in a generally vertical direction with a velocity of c2 where c2 is equal to the minimum significant media velocity to be considered. lf the times t1 and t2 are set equal to one another, then the cosine of the angle qb, Where cp is the helix angle, is equal to the minimum media velocity c2 divided by the velocity of steel c. Using the values `of 5000 feet per second as a lowest media velocity to be considered and 17,000 feet per second as the velocity of steel, the helix angle is calculated to be about 72.

A second path of sound transmission would be along the length of the housing parallel to the central axis of the housing. In this instance, the grooves serve to break up the straight line path lengthwise of the housing and, at low frequencies of acoustic impulses, the apparent velocity can be calculated from the Formula 5. For high frequency acoustic impuls-es, the transmission effects are minimized by keeping the sections of the housing 'wall between the grooves small with respect to the wave length of the acoustic impulses.

With only the foregoing construction, the housing would have an apparent velocity of about 12,000 feet per second. However, by means of the present invention, this apparent velocity can be further decreased by the filling or plugging of the outer groove 42 with a material 43 having a high density and a low modulus of elasticity. A suitable material is, for example, lead. The addition of material 43 effectively increases the mass and therefore decreases the apparent velocity. Attenuation of acoustic energy occurs by virtue of the acoustic impedance mismatch and the nature of the material 43 with a low modulus of elasticity characteristically having high attenuation.

In FIG. 7, another form which the housing 23' may assume is illustrated. In particular, housing 23', at the end of the acoustic inhibiting section, is reduced in diameter by means of a connecting stub member 35' -on which the transducer 35 is mounted. It will be readily apparent that mounting of the transducers in this manner can easily be accomplished by one skilled in the art. Housing 23 may be arranged similar to housing 23 and include an inner groove 41 and an outer groove 142. Groove 142 may be increased to a greater cross-sectional area than groove 42 of member 23. Hence, the larger cross-section permits the use of more loading material 43 with a greater decrease in apparent velocity than the 'elSocity decrease of the embodiment illustrated in FIGS.

l As shown in FIG. 4 the pattern of an acoustic inhibiting section can be modified in a somewhat different manner than the previously described patterns. The pattern which prevails over the length ofthe housing 23a between transducer sections (not shown) and as illustrated in FIG. 4 consists of groups 45 and 50 of cylindrical openings which alternate along the length of the housing. A first set or group 45 of openings includes eight openings 45a (FIG. 8) which are equidistantly spaced from one another about the periphery of the housings and respectively have their centers lying in a plane perpendicular t-o the central axis of the housing. Generally, a section 46 of the housing between adjacent openings along the perpendicular plane should be approximately equal to the diameter of an opening 45a. The second group 50 of openings similarly includes eight openings 50a which are equidistantly disposed about the periphery of the housing and have their centers lying in a plane perpendicular to the central axis `of the housing. To interrupt the linear acoustic path lengthwise of the housing, the second group 50 of openings is displaced angularly relative to a first group 45 of openings and has portions thereof disposed intermediate the space between openings in group 45 so that only thin interconnecting angularly disposed portions 47 of the housing remain. Housing portions 47 increase the length of acoustic path longitudinally ofthe housing. Each of the openings 45a and Sila receives a cylindrical plug 43a of lead which is suitably secured to the steel housing 23a.

Referring now to rFIG. 5, a pattern similar tothat described in connection with FIG. 4 is disclosed. In this pattern, the circular openings 55a in one group 55 have diameters twice as great as the diameters of the circular openings 69a of the second group 60. Only six openings about the periphery of theihousing are provided in each ofthe groups 55 and 60. vThe portions of the housing 23b which equidistantly space `the larger openings 55a from one another are substantially equal to .the diameter of the smaller openings69a while the portions of the housing which equidistantlyrspace the smaller openings oda from one another l are substantially equal to the diameter yof thelarger openings 55a. The axes of the larger openings of groups 55 are displaced at an angle of 30 from the axes yof the smaller openings of theV groups 60. The longitudinal displacement between adjacent planes on which the lcenters of the openings of the respective groups lie is substantially equal the sum of the radii ofa large and a small opening. Each of the openings 55a and 60a receive a cylindrical plug 43a of leadrwhich is suitably secured to .the steel housing 23h.

In FIG. 6, still another .conguration is illustr-ated wherein `a group 65 of. openings on a plane perpendicular to the llongitudinal axis of the housing includes a plurality of pairs .of adjacent openings 65a andyob wherein the pairs Vof -openings are spaced from one another about the periphery of theV housing byV-short section-s 66 of the housing. Each of the openings in a pair have central axes which respectively intersect the -central longitudinal axis of the housing and the spacing between adjacent openings `65a and 65b is less than the spacing between pairs such that there is relatively little metal extending longitudinally in a direct linebetween adjacent openings of 4a pair. The short sections 66 of housing which separate adjacent pairs of openings is ,approximately equal to. one 'half the diameters of an opening.

The group 65 of openings is repeated along the length of the housing but at different .angular orientations relative to the periphery of the housing. The different angular y orientation may be random or randomly systematic to disrupt the direct acoustic path lengthwise of the housing. Each of the openings 65a, 65!) receive a cylindrical plug 43a of lead which is suitably secured to the steel housing 23C.

lFrom the foregoing description `of the present invention it will be appreciated how =to provide an acoustic logging 'tool with a tubular housing `so constructed to substantially eliminate uniform longitudinal paths to inhibit the immediate tr-ansmission of detectable acoustic energy therealong wherein such a housingis loaded with a high density material having a low modulus of elastici-ty.

In the disclosed arrangements, the transducers are S arranged that they are exposed to well fluids. However, if desired, the housing :could be enclosed or plugged with .a rubber or other low velocity composition to provide a fluid tight housing. In such an arrangement, the interior of the Huid tight housing would be oil-filled for sound transmission purposes.

While particular embodiments of the present invention have been shown and described, it is apparent that changes .and modiiicaitions may be made without departing from this invention in its broader aspects and, therefore, the .aim in the appended claims is to cover all .such changes :and modications as fall within the true spirit and scope of this invention.

We claim:

1. Apparatus for use in well bore-s comprising: an elongated, rigid tubular member of .a material having a relatively high characteristic .acoustic velocity; at least two acoustic transducers in 4a xed, spaced relationship relative to Vone another along a section ofsaid tubular-member; said tubular -member having a coniiguration between .said acoustic transducers arranged to alter the intensity and time of `arrival of detectable acoustic energy traveling 4through and longitudinally of said tubular member, such configuration including interruptions in Vthe longitudinal ,Continuity of said tubular member to provide tortuous .said member yand being -adequateto changethe weight of :thevtubular memberindependently of its unit spring .rate and thereby further alter the ,intensity and time of .arrival .of detectable .acous-ticrenergy.

2. Apparatus for use in well bores comprising: an

elongated, rigid, Itubular member of a material having a relatively high characteristic .acoustic velocity; at least two lacoustic transducers in a xed, spaced relationship -relative .to one another along a section of said tubular mem- -berg said tubular member having a conguration between said acoustic transducers arranged to alter the intensity -rand time of `arrival of detectable acoustic energy travel- -ingthrough and longitudinally of said` tubular member;

such conguration including perforations along the length of said tubular member substantially interrupting the longitudinal continuity lof .said tubular member, and material filling :said perforations, said material having a high density and, low modulus of elasticity relative .to the mate- `rial of said .member and being `adequate to change the weight .of the tubular member independently of its unit spring rate and thereby further alter `the intensity and `time of arrival of detectable acoustic energy.

3. Apparatus for use in well bores comprising: an

elongated, rigid, Itubular member of steel; at least two .acoustic transducers in a xed, spaced relationship relative to one another along a section of said tubular mem- Yber; said tubular member having a coniiguration between said acoustic transducers arranged to yalter the intensity and time of arrival of detectable .acoustic energy travelying through and longitudinally. of said tubular member;

such configuration including interruptions in the longitudinal continuity of sai-d tubular member forming a series ,of interlinking metallic segments providing tortuous paths for passage of acoustic energy which paths `are longer than the longi-tudinal spacing between said transducers, and material secured to said interruptions in said tubular member, said material having a high density `and low modulus of elasticity relative t-o the material of said member `and being adequate to `change the weight of the tubular member independently of its unit spring rate and thereby further alter the intensity and -time of arrival of detectable acoustic energy.

4. Apparatus for use in well bores comprising: an elongated, rigid, tubular member of steel; at least two acoustic transducers carried in a iixed, spaced relationship relative -to one .another along a section of said tubular member; said tubular member having a configuration between said acoustic transducers arranged to alter the in- .tensity and :time of arrival of detectable acoustic energy traveling through and longitudinally of said tubular memlber; such configuration including interruptions in `the longitudinal continuity of said tubular member'forming a series of interlinking metallic segments providing tortuous paths for passage of acoustic energy which paths are longer Ithan the longitudinal spacing between said transducers, the width of such paths being a quarter wave length tor less than that of the predominant Ifrequency of an .acoustic signal traveling between said transducers, said interruptions serving to alter a relationship between the unit spring rate of said tubular member and the weight per unit of length so `as to increase the transit time of detectable acoustical energy between said transducers while maintaining the rigidity of said tubular member with lll respect to bending moments, and material secured to said .tubular member in said interruptions, said material having .a high density 4and a low 4modulus of elasticity relative to the material of said member and being adequate to change the weight .of the tubular member independently of its unit `spring rate and thereby further .alter the intensity and time of arrival of detectable acoustic energy.

y5. Apparatus for use in well bores comprising: an elongated, rigid, tubular member of steel; at least two acoustic transducers carried .in a fixed, spaced relationship relative to one another along a section of said tubular member; said tubular member having a configuration between said acoustic transducers arranged to alter the intensity and time of arrival of detectable acoust-ic energy traveling through and longitudinally -of said tubular member; such configuration including interruptions in the longitudinal continuity of said tubular member yforming `a series of interlinking metallic segments providing tortuous paths for passage `of acoustic energy which paths are longer than the longitudinal spacing between said transducers, the width of such paths being a quarter wave length or less than that of the predominant frequency of an acoustic signal traveling between said transducers, said interruptions serving to increase t-he travel time of detectable acoustic energy traveling between said transducers in accordance with the formula where t equals time, S equals the distance between transducers, g equals the gravitational constant, s equals the unit spring rate vand w equals the weight per unit of elongation, and material secured to said member in said interruptions, -said material having a high density and a low lmodulus of elasticity relative to the material of said member .and being adequate to change the weight per unit of length independently of the unit spring rate and thereby further alter the intensity and time of arrival of detectable .acoustic energy.

I6. Apparatus for surveying well bores comprising: an

elongated, rigid, metallic, tubular member; at least two acoustic transducers supported by said tubular member, -said 4tubular member having a configuration between said acoustic transducers arranged t-o alter the intensity and time of arrival of detectable acoustic energy traveling through and longitudinally of said tubular member, such configuration including recesses extending inwardly from the inner and outer walls of said tubular member so as to interrupt the linear continuity of the wall of the tubular member .and lthereby provide a tortuous path for acoustic energy traveling along said tubular member, such recesses thereby reducing the amount of metal in said tubular member so that the relationship between the unit spring rate and weight per unit of length substantially determine Ithe acoustic velocity for acoustic energy traveling the distance between lsaid transducers, the width of such path being a quarter wave length or less than that of the predominant frequency of the acoustic signal to attenuate acoustic ysignals with a lower frequency than the predominant frequency, and material secured to said recesses, said material having a high density and a low modulus of elasticity relative to the material of `said member and being adequate to change the weight of the tubular member independently of its unit spring rate and thereby further alter the intensiy and time of arrival of detectable acoustic energy.

References Cited by the Examiner UNITED STATES PATENTS 1,678,116 7/28 AHarrison 340-17 2,648,056 8/53 Jakosky. 2,834,421 5/58 Summers 18l--.5 2,938,592 5/60 Charske et Ial 181-.5 2,994,398 8/61 Engle etal 181-.5 3,054,471 9/ 62 Knudsen 181-.5

FOREIGN PATENTS 163,459 6/55 Australia.

SAMUEL FEINBERG, Primary Examiner.

ARNOLD RUEGG, KATHLEEN H. CLA=FFY,

Examiners. 

1. APPARATUS FOR USE IN WELL BORES COPRISING: AN ELONGATED, RIGID TUBULAR MEMBER OF A MATERIAL HAVING A RELATIVELY HIGH CHARACTERISTIC ACOUSTIC VELOCITY; AT LEAST TWO ASCOUSTIC TRANSDUCERS IN A FIXED, SPACED RELATIONSHIP RELATIVE TO ONE ANOTHER ALONG A SECTION OF SAID TUBULAR MEMBER; SAID TUBULAR MEMBER HAVING A CONFIGURATION BETWEEN SAID ACOSTIC TRANSDUCERS ARRANGED TO ALTER THE INTENSITY AND TIME OF ARRIVAL OF DETACHABLE ACOUSTIC ENERGY TRAVELING THROUGH AND LONGITUDINALLY OF SAID TUBULAR MEMBER, SUCH CONFIGURATION INCLUDING INTERRUPTS IN THE LONGITUDINAL CONTTINUITY OF SAID TUBULAR MEMBER TO PROVIDE TORRUOUS ALTERNATIVE PATHS FOR PASSAGE TO ACOUSTIC ENERGY, WHICH PATHS ARE LONGER THAN THE LONGITUDINAL SPACING BETWEEN SAID TRANSDUCERS, AND MATERIAL SECURED TO SAID MEMBER IN SAID INTERRUPTIONS, SAID MATERIAL HAVING A HIGH DENSITY AND LOW MODULUS OF ELASTICITY RELATIVE TO THE MATERIAL OF SAID MEMBER AND BEING ADEQUATE TO CHANGE THE WEIGHT OF THE TUBULAR MEMBER INDEPENDENTLY OF ITS UNIT SPRING RATE AND THEREBY FURTHER ALTER THE INTENSITY AND TIME OF ARRIAVE OF DETACHABLE ACOUSTIC ENERGY. 